Frequency diversity telegraph system



345 H. A. RHDE I FREQUENCY DIVERSITY TELEGAPH SYSTEM Filed June 50, 1943 INVENTOR E/lwdes Patented Apr. 2, 1946.

GFFICE SITY TELEGBAPH SYSTEM Harold Anson Rhodes, Bloomfield, N. J., assignor to American Telephone and Telegraph Com pany, a corporation of New York nppueauon June an, 1943, serial No. 492,787

(ci. irs-ss) 11 Claims.

lIlhis invention relates generally to signaling systems, and more particularly to telegraph systems. Still more particularly, this invention relates to two-tone telegraph systems, as well as to frequency diversity telegraph systems employing either wire line, radio or other transmission media. l Telegraph signals are often transmitted either by interrupting the current of a battery or other source in accordance with the vsignals or by reversing the polarity of the current of the source, or by modulating an audio-tone by D. C. signals. In these types of transmission the transmitting medium may be such that the signals may be greatly attenuated and sometimes lost. It has been proposed to overcome the effect of the attenuation characteristic of the transmitting medium by employing a two-tone method for conveying the signals. In the two-tone method current of one frequency is produced for a marking signal and current of anotherr frequency for a spacing signal. In order to reproduce the signais transmitted by the two-tone method, it has been proposed to employ s. limiter at the receiver to substantially reduce the effect of the attenuation characteristic of the transmitting medium.

However, when one of the currents, such as the marking current, becomes attenuated or faded,

- the limiter will introduce noise and extraneous currents and transmit these undesired currents to the telegraph receiving relay. Consequently the signals maybe distorted and perhaps rendered unintelligible.

In accordance with this invention it is proposed` to transmit telegraph signals over a transmitting medium having high attenuation by a frequency diversity two-tone method. By this method currents of two discrete frequencies will be produced for each single marking signal, and currents of two discrete but different frequencies will'be produced for each spacing signal. At the receiving station of this system the currents of both the marking and spacing frequencies will be received and amplified. One limiter will be arranged to act on one of the marking frequencies and one of the spacing frequencies and another limiter will be arranged to act on the other marking frequency and the other vspacing frequency. Each limiter will thus act on thecurrents of the two frequencies normally assigned to it in such a manner as to materially reduce the effect of noise and other extraneous currents. During periods of selective fading when only one of the two marking pulses. for example, may be reduced to or bewill be present at the receiver in suiilclent magnitude for properly actuating the telegraph reoeivlB ing relay.

Another feature of this invention is the provision of means for coupling the circuits of the two limiters to each other for further overcoming the effects of noise and other extraneous currents. The currents of the different frequencies received w.ll be divided into two bands. namely, upper and lower bands, and each of the two bands of fre quencies will be transmitted over a separate path. Each path will include a limiter, filtering apparatus and detecting apparatus. 'I'he equipment in each path will normally act on the currents of the band of frequencies assigned to it so as to rep:oduce the received signals. However, the two limiterslwill be electrically connected or coupled to each other in this invention, as already' stated. If the lower band of frequencies, for example, Abecomes attenuated due to poor conditions of transmission, part of the higher band of frequencies will be diverted from its own path through the coupling circuit to the path for the lower band of frequencies. The limiter in the path of the lower frequencies will act on the higher band of frequencies received over the coupling circuit and greatly reduce the effect of noise or other extraneous currents in that path.` After discriminating against noise and other extraneous currents, the filtering apparatus following the limiter in the lower band path will substantially suppress the currents of the higher frequencies which were acted on by the limiter of the lower frequency path. In other words, currents in the good path will be diverted to the limiter of the poor path to overcome noise during adverse conditions of transmission and the diverted currents L for one signaling channel. When the key K is closed for the purpose of producing a marking pulse or signal, the lower windings of relays R1,

low the noise level at the receiver, the other pulse R2', Ps and Rs will be energized so as to cause the then transmitted through the limiter IML armatures of these relays to move to the right to close their marking contacts M. The opening of the spacing contacts S of relays R1 and Rs will remove the shunts around oscillators Oi and Oz so that the oscillators 01 and O2 may transmit currents of their individual frequencies to the sending filters SFr and SF2, respectively. At the same time the closure of the marking contacts M of relays Rs and Re will provide shunt circuits around the oscillators Oz and O4 and, therefore,

substantially suppress any currents that may be generated by these oscillators. When the key K is vopened for the purpose of producing a spacing signal or pulse, the armatures of the relays Ri in accordance with a marking signal, and curr` rents of two different frequencies corresponding to oscillators Oa' and O4which are also different from the frequencies of oscillators l and Oss-- will be transmitted in accordance with a spacing signal. n order to better understand the inven tion, it maybe stated that oscillators Oi and 0n may produce currents of, for example, 765 cycles per second and 1785 cycles per second, while the oscillators Oa and O4 may produce, for example. currents of 935 and 1955 cycles per second. These frequencies are mentioned only for illustration, and other frequencies widely different from those mentioned may be used within the scope of this invention. The sending bus BSi will, therefore, receive either the two marking frequencies of oscillators O1 and Oz, or the two spacing pulses of oscillators Os and O4, along with the marking and spacing currents of' still other frequencies.

produced in other` channels which are connected to the same bus BS1. The currents reaching the bus BSi will be amplified by amplifier Ai, and then may be impressed upon a radio transmitter RT having an oscillator O used for modulating the signaling frequencies to provide.the much higher frequencies required for radio transmission via antenna AN.

Currents received by the receiving antenna HLA` at the receiving station are demodulated with current of an oscillator Os to reduce the received radio frequencies to their original positions in the frequency spectrum. Currents of the demodulated frequencies will be amplified by amplifier A2 and impressed upon the receiving bus BSz. The receiving filters RF1 and RF: connected to the receiving bus BS: select and transmit only the currents of the marking frequencies generated at the transmitting station by oscillators O1 and O2, respectively. The receiving filters RFz and RF4 connected to the receiving bus B52 select and transmit only the currents of the spacing frequencies generated at the transmit ting station by oscillators O3 and O4, respectively,

The outputs of the filters RF1 and RF: are than 'combined and amplified by amplifier As, and are The lters RFs and RFe connected to limiter LMi separate the marking and spacing frequencies from each other, and at the sa'me time suppress harmonies and other eects produced by the limiter aaezeea LMr itself. The currents transmitted by filters RFs and RFS are amplified and detected by the apparatus AD1 and ADa, respectively, and actuate the relay Ro so as to move its armature to its RF1 and RFS, which separate the other markingv and spacing frequencies from each other and at the same time eliminate harmonics and other extraneous currents introduced by the limiter LMz. The filters RF1 and RFs are connected to the amplifier-detectors AD: and AD4, respectively, which transmit the marking andspacing currents to the telegraph receiving relay Ro, causing the armature of this relay to move to its marking contact M or its spacing contact S in accordance with the received signals. The armature of the relay Ro may be employed to control the operation of a teietypewriter TTY, or a recorder of any well-known type, or any other signal indicating apparatus.

Whenever one of the frequencies, such as one of the marking frequencies of, for example, 765 cycles, becomes substantially reduced in amplitude due to fading or other properties of the transmission medium, current of the other marking frequency of 1785 cycles conveying the same marking signals will be present in the limiter LMa. Consequently the marking signal will be transmitted to the receiving apparatus including the receiving relay Ro. The limiters LM; and LMz will, therefore, effectively select the strongerv of the two currents of the marking frequencies and transmit they currents of that frequency to the practical exclusion of noise and other extraneous currents. Likewise the limiters LMi and LMz will effectively transmit the stronger of the two currents of spacing frequencies and eliminate noise and other effects, In general, therefore, the receiving relayRo will be operated to respond to the marking and spacing signals, and the effect of noise and attenuation changes in the system will be considerably reduced. This is one of the features of this invention,

At times one of the marking currents of, for example, 765 cycles, may be absent from one of the paths of the receiving circuit due to adverse conditions of the transmitting medium. During any such occasions the amplifier Aa will receive from amplifier A4 of the other path some of the marking current of 1785 cycles. The 1785 cycle marking current will be received by amplifier Aa over the 'coupling circuit CC which has a low, but predetermined loss. The 1785 cycle marking'current will then be limited by limiter LM; so as to substantially reduce the effect of noise and extraneous currents. The 1785 cycle marking current will thereafter be suppressed by filters RFS and RFe which cannot transmit the higher frequency marking current of 1785 cycles. Consequently no current will then be detected by the marking detector ADi (or by the spacing detector ADa). The receiving relay Ro will therefore respond tc the marking current of 1785 cycles as it is detected normally by detector ADa. This arrangement will thus prevent the false operation of relay Ro when the '765 cycle marking current is faded considerably.

If both the marking frequency of 765 cycles and the spacing frequency of 935 cycles are faded, then the coupling circuit CC will divert to amplier A3 some of the higher frequency marking :urrent` of 1785 cycles and some of the higher :'requency spacing current of 1955 cycles. These :urrents will then be alternately acted on by imiter LM1 to`- overcome noise and other unde- ;ired currents. These higher frequency currents yill thereafter be suppressed by lters RFs and RFB. If the higher frequency currents are atteniated by the transmitting medium, the coupling :ircuit CC'will likewise divert some of the corresponding lower frequency currents from the amnliiier Ae to the amplifier A4 for the same purpose, is will be readily apparent.

Without the coupling or connection' of the two :hannels or paths by the coupling circuit CC as ;hown and just described, the absence of the cur- :ent of one of the-spacing frequencies of, for eximple, 1955 cycles will cause the limiter LMa to zransmit noise currents to the spacing ilter RFa,

may also be limited by the other limiter, ltering apparatus connected to one of the limiters for transmitting only the currents lying within said first frequency range, -and separate filtering apparatus connected to the other of the limiters for transmitting only the currents lying within said second frequency range.

2. In a frequency diversity signaling system in which signals are transmitted by means of currents positioned in one frequency range and the same signals are simultaneously transmitted by which in turn will be conveyed to the receiving :elay R to cause it to operate falsely. The cou- ;ling of the two channels,or paths greatly im- Jroves the operation of the system and aids in ',he elimination of erroneous signals. i

Although the amplifiers A3 and A4 are shown :oupled to each other for the purpose of eliminatng noise and other such effects, it will be under- ;tood that the limiters LM; and LMn may be so zoupled, if desired, to produce thev same result. Fo a'iomplish this same end by coupling the lim ters LMi and LMz, it will be necessary simply to :onnect or couple the input circuits of the limiters .o each other by a coupling circuit such as CC or n any well-known manner.

By producing two or more currents of dierent :"requencies for the transmission of a signal, such is a marking signal, and two or more similar currents of different frequencies for the transmis- ;ion of a spacing signal, and having a limiter act Jn thesev currents as already described, a substantial improvement in transmission has been abserved. A further distinct improvement has Jeen observed by coupling of the two channels to each other at the receiver, as alreadydescribed.

Although a key K is shown for producingy the parking and spacing signals at the transmitter nerely for illustrative purposes, it will be understood that the signals transmitted may be proiuced by a teletypwriter or by any other means.

Although the frequency diversity system of this .nvention has been shown and described in con` section with a radio system, the invention is equally applicable to systems vtransmitting signals over land lines or other media. Although the .nvention has been shown in regard tothe transnission of signals from one point to another, it .s equally applicable to two-way transmission.

While this invention has been shown and described in certain particular embodiments merely for the purpose of illustration, it will be understood that the general principles of this invention nay be applied to other and widely varied or` ganizations without departing from the spirit of the invention and the scope of the appended zlaims.

What is claimed is:

l. In a frequency diversity signaling system which signals are transmitted by means of currents positioned in a first frequency range and she same signals are simultaneously transmitted ay means of currents positioned in a second freiuency range, the combination of two current .imiters which separately receive the currentsl Nithin said iirst and second frequency ranges,

means of currents positioned in a different frequencyrange, the combination of two ampliiiers for separately amplifying the currents of the two different frequency ranges, two limiters con-. nected respectively to said amplifiers for limiting the amplitudes of the currents amplified by said amplifiers, means for coupling the output circuits of said amplifiers to each other so that the amplified currents limited by one of the limiterspiiay also be limited by the other limiter, filtering apparatus connected to one of the limiters for transmitting only the currents amplified lby the amplifier to which said limiter is connected, and separate iiltering apparatus connected to the other of the limiters for `transmitting only the currents amplified by the amplifier to which the latter limiter is connected.

3. In a telegraph system in which a marking signal is transmitted by means of a plurality of currents at different frequencies and a spacing signal is transmitted by means of a plurality of currents of different frequencies, the combination of two amplifiers, one of which amplies certain of said currents, the other amplifier amplifying the remaining currents, two limiters, two filters, one

and spacing signals.

4. In a frequency diversity telegraph system in which each marking signal and each spacing signal is transmitted by means of a plurality of ,i

currents of different frequencies, the combination of two amplifiers, two limiters connected to said amplifiers, respectively, one of the ampliy fiers and `its associated limiter acting on certain of the currents corresponding to each marking signal and each spacing signal while the other amplifier and limiter acts onthe remaining currents corresponding to each marking signal and each spacing signal, filtering means con` nected to said limiters, common detecting apparatus connected to said filtering means for reproducing the signals, said amplifiers being directly coupled. to each other so that all of they currents corresponding to the marking and spacing signals may be applied to each limiter.

5. .In a telegraph system in which two alternating currents of different frequencies are employed to transmit a marking signal and two alternating currents of different frequencies areI employed to transmit a spacing signal, the combination of a first limiter for limiting one of 'the currents corresponding to each markingsignal and one of the currents corresponding to` 3 each spacing signal, a second limiter for limiting the remaining currents corresponding to each marking signal and each spacing` signal, rst and second filters connected, respectively, to said ylimiters for selectively transmitting the currents acted on by said limiters, common detecting ap` paratus connected to said filters for reproducing the marking and spacing signals, Yand means for coupling said rst and second limiters to each other so that the first limiter may act on the currents transmitted to the second limiter and the second limiter may act on the currents transmitted to the first limiter.

6. In a telegraph system in which each marking signal is transmitted by means of currents oftwo different frequencies and each spacing signal is transmitted by means of currents of two other frequencies, the combination of receiving apparatus for receiving said signals, said receiving apparatus including a detector for reproducing said marking and spacing signals,'said detector being supplied with the currents to be de- A tested over two dierent paths, each path including a limiter, a band'pass lter connected to the output of the limiter, and means for coupling the limiters of the two paths to each other.

'7. In a. telegraph system, the combination of first and second limiters, said limiters receiving alternating currents corresponding to a marking signal and other alternating currents of different frequencies corresponding to a spacing signal, rst and second filters connected, respectively, to said limiters, the first filter transmitting only the lowerA frequency currents corresponding to the marking and spacing signals, the second filter transmitting only the higher frequency currents corresponding to the same marking and spacing signals,detecting apparatus connected to both of said filters, and means for coupling said limiters to each other so that both limiters may act on the same alternating currents.

8. In a telegraph system in which two cur-v rents of dierent frequencies are employed to producing said marking and spacing signals, and means for coupling said limiters to each other so that each limiter may limit the currents transmitted through the other limiter.

9. In a telegraph system in which a current of low frequency and a current of high frequency are employed to transmit each marking signal and another current of a different low frequency and another current of a different high frequency are employed to transmit each spacing signal, the combination of rst and second limiters for separately limiting the amplitudes of the low frequency currents and the high frequency currents,

flow and high pass filtering apparatus connected respectively to said rst and second limiters for separately iiltering the low frequency currents and the high frequency. currents, means comprising a predetermined low loss path for coupling said limiters to each other so that both limiters may limit the amplitudes of the currents of the same frequencies, and means coupled to said filtering apparatus for reproducing said marking and spacing signals.

10. In a signaling system for telegraph signals in which each marking signal and each spacing signal is conveyed by a plurality of alternating currents, the combination of two lters for separating said currents into tworbands occupytransmit a marking signal and two currents of vdifferent frequencies are employed to transmit a spacing signal, the combinationof first and second limiters for receiving, respectively, two portions of the currents corresponding to the marklirequency ranges, the second filter transmitting only the currents in the other frequency range, detecting means connected toI both filters for reingdiiferent ranges in the frequency spectrum, two amplifler-limiters connected respectively to said two filters, means for coupling saidamplifier-limiters to each other, two additional filters of the same frequency characteristics as said first filters and connected respectively to said amplifier limiters for receiving-the limited currents, and means coupled to said amplifier-limiters to reproduce said signals. ,Y

1l. In a signaling system in which a plurality of currents of different frequencies are employed to transmit each of the signals, the combination of two paths including means for receiving the low frequency currents and the high frequency currents respectively, each path includingia limiter by a filter, the filter of each path passing only the currents having thefrequencies received by that path, a low loss circuit coupling the limiters of the twopaths,l and means connected to said filters for reproducing the. signals.

HAROLD A. RHODES.` 

